Prior to the advent of large scale use of polymers in such applications as continuous film or filament production, the needs of industry for precision blending of bulk solids products were met with mechanical tumbler, ribbon or screw blenders. Capacities of these units ranged from one cubic foot to over 1,000 cubic feet.
As the demand for plastics grew, it became apparent that much larger blender volumes were necessary to allow continuous production lines in plastics users' plants to operate without frequent shutdowns caused either by (1) variations in physical properties or (2) additive content inherent in the producer's production processes. This led to a demand for tumble blenders in the nominal 6000 cubic foot capacity range.
The high cost of large tumble blender installations prompted industry-wide efforts to develop a blending capability in storage silos to comply with the product uniformity requirements of the polymer industry. A number of designs resulted, some silo blenders having capacities in the 30,000 cubic foot range.
As storage bins or hoppers are filled with granular or particulate material, it often happens that an inhomogeneous distribution of material occurs. There may be several reasons for this result. In the first place, as material flows into a hopper, the material beneath the inlet nozzle piles up at the angle of repose of the material. In this case the larger particles often roll down the peak toward the sides of the hopper, leaving the finer particles in the central region. Inhomogeneity can also occur when the hopper is filled with different batches of the same material because of variations of composition of individual batches. When material is drawn off through an outlet at the bottom of the hopper, the material flows from the region directly above the nozzle. Thus the material will not be representative of the average characteristics of the material in the hopper.